Delete links to lessons on dot org

There are links to Silverstripe lessons on silverstripe.org which are now very old. These links should be removed.

### Notes
- We are not getting rid of the lessons themselves on silverstripe.org at this stage

### Acceptance criteria
- [x] Remove any links to the lessons on silverstripe.org on the CMS 6 branch

## PRs
- https://github.com/silverstripe/developer-docs/pull/568
